Diamond has many extraordinary properties, including superlative hardness, thermal conductivity, and optical transmissivity. Synthetic diamond produced by chemical vapor deposition ("CVD") has become commercially viable for practical applications such as wear parts, heat sinks, and optical windows. However, while the cost of producing CVD diamond has decreased in recent years, it is still quite expensive.
The production of diamond film in a chemical vapor deposition process, such as a plasma jet CVD process, involves consideration of many practical, as well as technical, factors. In order to obtain the relatively high yield that is necessary for cost effectiveness, the process is carried out at high temperatures. The large heat fluxes at the deposition region during and after deposition cause stresses in the diamond that can result in cracking of the diamond film and/or lifting of the film from the deposition target medium before deposition is complete. When attempting to deposit relatively thick films (for purposes hereof, at least 100 microns thick, and, for many applications, greater than 500 microns thick) the problems of film cracking and/or premature lifting (also called delamination) can be particularly vexing, and can reduce production yields and prevent cost effective operation. So-called "repeatability" is also a problem; that is, the ability to obtain consistent results from ostensibly the same operating conditions that proved successful on one or more occasions.
It has been recognized that a source of stress that can crack and/or prematurely delaminate a diamond film is a mismatch between the coefficients of thermal expansion of the diamond and the target medium upon which it is being deposited. To address this problem, deposition substrate materials having coefficients of thermal expansion relatively close to that of diamond can be selected. However, in selecting a substrate material, other properties must also be taken into consideration. For example, the material must be able to maintain its integrity in difficult environmental conditions of deposition, which include a high temperature and the presence of reactive substances, such as the atomic hydrogen that is essential for the diamond deposition process. As an example, graphite is attractive as a substrate material because its coefficient of thermal expansion is generally close to that of diamond. However, atomic hydrogen attacks graphite. One solution in the prior art has been to coat the graphite with a thin coating of a material such as molybdenum or tungsten, or carbon-containing compounds such as silicon carbide. These approaches have met with only limited success in improving the yield of relatively thick intact diamond films. The powders of a wide variety of substances (for example, fine powders of SiC, Si, Mo, W, Al.sub.2 O.sub.3, Ti, Ta, TiO.sub.2, h-BN, c-BN, SiO.sub.2, B.sub.4 C, AlN, Si.sub.3 N.sub.4, WC, MoC, or MoS.sub.2, taken alone or in combination) have been proposed as coatings for many different substrate materials, and have apparently exhibited some success, at least when producing relatively thin diamond films. Reference can be made to U.S. Pat. No. 5,180,571. However, there is still much room for improvement, particularly with regard to yield and repeatability when making thick films.
It is among the objects of the present invention to devise a technique and a deposition target medium that improve the production of synthetic diamond film by facilitating intact growth of relatively thick diamond films by CVD methods, especially high heat flux CVD, such as CVD plasma jet deposition, and, to a lesser extent, by also facilitating release of the fabricated intact diamond film after deposition.